J-20... The New Generation Fighter II

Re: Final estimate for J-20 canards' radar return energy is 3.276x10^-19

no_name said:

He didn't mention the LERX. Maybe it was not obvious to him.

J-20 design combine canards, LERX and delta wing together to generate alot of extra lift. What this may imply is that you do not need to raise the aircraft into an as high AoA compared to other aircrafts for the same lift force. This I think means less drag and energy bleeding for maneuvering.

edit: however if you have a longer aircraft the cross sectional area will be present to the airflow for less AoA.

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you are right, J-20 had been used many advanced design , CAC considered that China engine technology deficiencies, such design may generate alot of extra lift, less drag, increase maneuvering. but the new design needs more flight test, to solve more difficulties. if China engine technology gets breakthrough, WS-15 develop smooth and success, that J-20 will be an excellent 4 generation fighter in the world.

nice pictures below:

Re: Final estimate for J-20 canards' radar return energy is 3.276x10^-19

Bltizo said:

His assessment that the J-20 will be used for anti ship purposes are dashed by the simple fact that there are no AShMs at the moment which can fit internally and still have good range and manouverability.

And if it were an uncompromising supercruiser/missile carrier then why not go for a clean delta rather than canard delta?

Then there was He Weirong's statement of 4S, with supermanouverability one of the key aspects of the 4th generation fighter -- and I'm sure the PLAAF recognize the value of agility, with years of J-10 and flanker experience, I can't imagine they would let go of that for their new premier fighter craft and modify it instead for a role usually relegated to naval aviation.

The article has a good deal of technical and professional sounding words, but I question if the writer knows what he is talking about. Can anyone familiar with aerodynamics give a rundown on the article?

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One point I can say is that I think his way of guessing at the center of gravity is odd. My way would be to guess that the nose wheel carries 15 % of the weight, the main wheels 85 % when the aircraft stands and determine the position of the center of gravity accordingly.

Can a F-22 detect a J-20 beyond visual range? Probably not.

The F/A-22's radar contains 2,000 T/R modules with a power output of 4 Watts each. The total power output of the F/A-22 AN/APG-77 radar is 8,000 Watts or 8 KW.

The maximum theoretical power that may return to the radar receiver after scattering off the J-20's canards is:

8 KW x 1.035 x 10^-17 = 8.28^-13 Watts or 0.000000000000828 Watts.

The extremely tiny 8.28^-13 Watts of radar energy return will most likely be absorbed by the intervening air, water, dust, clouds, soot, etc. This is true, because we know that "the atmosphere is not perfectly transparent at any radio frequency."

Since military equipment specifications are highly classified, I have no choice and have to make reasonable assumptions to gain insights.

Here, I will assume that the F/A-22 AN/APG-77 radar has the same ballpark performance as the EADS DR 174 3D radar. My previous calculation has shown that a 20 KW DR 174 can detect a F-22/J-20 at 13.5 km. Since the F/A-22 AN/APG-77 has only 8 KW, we expect the F-22 to detect an incoming J-20 at roughly 5.4 km (e.g. 13.5 km * (8 KW/20 KW) = 5.4 km).

The bottom line is that the J-20 is a very big deal. It looks like the F-22 will have to engage the J-20 in a dogfight. In this situation, assuming training is roughly equivalent, the side with the greater number of stealth fighters should have a decisive edge.



"3.0 Technology

The F/A-22's AN/APG-77 radar is an active-element, electronically scanned (that is, it does not move) array of around 2000 finger-sized transmitter / receiver modules. Each module weights ca 15g and has a poweroutput of over 4W. The APG-77 is capable of changing the direction, power and shape of the radar beam very rapidly, so it can acquire target data, and in the meantime minimizing the chance that the radar signal is detected or tracked."



"There may be more water in the air after a rainstorm, or near the ocean. .... shows occur when the air contains many small particles of dust or water. ... protects us by absorbing and scattering harmful radiation from the sun and space. ..."



"The atmosphere is not perfectly transparent at any radio frequency. The figure below shows how the zenith (the direction directly overhead) opacity z varies with frequency during a typical summer night in Green Bank, WV, with a water-vapor column density of 1 cm, 55% cloud cover, and surface air temperature T=288 K=15 C. The total opacity is the sum of several components (Leibe, H. J. 1985, Radio Science, 20, 1069):

(1) The continuum (broadband) opacity of dry air results from viscous damping of the free rotations of nonpolar molecules. It is relatively small (z001) and nearly independent of frequency.

(2) Molecular oxygen (O2) has no permanent electric dipole moment, but it does have rotational transitions that can absorb radio waves because it has a permanent magnetic dipole moment. The atmospheric-pressure-broadened complex of oxygen spectral lines near 60 GHz is quite opaque (z1) and prevents ground-based observations between about 52 GHz and 68 GHz.

(3) Hydrosols are liquid water droplets small enough (radius 01 mm) to remain suspended in clouds. Since they are much smaller than the wavelength even at 120 GHz (25 mm), their emission and absorption can be described by the Rayleigh approximation and their opacity is proportional to −2 or 2.

(4) The strong water-vapor line at 22235 GHz is pressure broadened to 4 GHz width. The so-called "continuum" opacity of water vapor at radio wavelengths is actually the sum of line-wing opacities from much stronger water lines centered on infrared wavelengths. In the plotted frequency range, this continuum opacity is also proportional to 2. Both the line and continuum zenith opacities are directly proportional to the column density of precipitable water vapor (pwv) along the vertical line-of-sight through the atmosphere. The pwv is conventionally expressed as a length (e.g., 1 cm) rather than a true column density (e.g., 1 gm cm−2), but the two are numerically equivalent because the density of water is one in cgs units."

Shouldn't the strength of the return signature of a radar beam be dependent on intensity? A radar beam with the same power but more focus would generate a much stronger return signature, so that how well an array can detect should be dependent on how well it can focus its individual modules as opposed to just how much power it can output?

latenlazy said:

Shouldn't the strength of the return signature of a radar beam be dependent on intensity? A radar beam with the same power but more focus would generate a much stronger return signature, so that how well an array can detect should be dependent on how well it can focus its individual modules as opposed to just how much power it can output?

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To answer your point directly, I don't think you can focus the beam until you have first detected the enemy fighter. If you knew where the enemy fighter was located then indeed, you can focus the entire radar beam on the fighter. However, you can't acquire that information unless you put the radar in a wide scanning mode.

To generate a higher beam intensity, you can reduce the scan area for the emitting radar. The current assumption is that the emitting radar is scanning an area that is 1 km in radius. You can reduce the radius to 0.5 km to increase the radar intensity. Even if you reduce the scanning area from 1 km radius to 0.5 km or 0.25 km, the outcome is still an almost undetectable return signal at beyond-visual-range distances.

In any case, I did not want to pursue a refinement of the estimate for the scan area. It seems apparent that the radar return is virtually undetectable. If you want to refine the model, you would have to consider that a fighter needs to scan forward in a semicircle (e.g. 180 degrees). Next, you have to determine the height of the scan area (e.g. from the ground/"terrain-hugging attack" to the maximum aircraft ceiling height). That will give you a cylindrical surface.

After that, you pick a reasonable distance (e.g. 13.5 km or more). Finally, you estimate how long it will take to scan the entire cylindrical area in a 1- km, 0.5- km, or 0.25 km-radius surface-area circle. If I knew the scan rate of the AN/APG-77 then I would perform the calculation. However, I do not know this important specification. Hence, a further calculation and refinement will still only be a guess and not worth the trouble.

The calculations that I have already provided have yielded approximate detection ranges. There is a diminishing return in building more complex models and attempting to refine the estimates. You can only go so far with open-source material.

Martian said:

To answer your point directly, I don't think you can focus the beam until you have first detected the enemy fighter. If you knew where the enemy fighter was located then indeed, you can focus the entire radar beam on the fighter. However, you can't acquire that information unless you put the radar in a wide scanning mode.

To generate a higher beam intensity, you can reduce the scan area for the emitting radar. The current assumption is that the emitting radar is scanning an area that is 1 km in radius. You can reduce the radius to 0.5 km to increase the radar intensity. Even if you reduce the scanning area from 1 km radius to 0.5 km or 0.25 km, the outcome is still an almost undetectable return signal at beyond-visual-range distances.

In any case, I did not want to pursue a refinement of the estimate for the scan area. It seems apparent that the radar return is virtually undetectable. If you want to refine the model, you would have to consider that a fighter needs to scan forward in a semicircle (e.g. 180 degrees). Next, you have to determine the height of the scan area (e.g. from the ground/"terrain-hugging attack" to the maximum aircraft ceiling height). That will give you a cylindrical surface.

After that, you pick a reasonable distance (e.g. 13.5 km or more). Finally, you estimate how long it will take to scan the entire cylindrical area in a 1- km, 0.5- km, or 0.25 km-radius surface-area circle. If I knew the scan rate of the AN/APG-77 then I would perform the calculation. However, I do not know this important specification. Hence, a further calculation and refinement will still only be a guess and not worth the trouble.

The calculations that I have already provided have yielded approximate detection ranges. There is a diminishing return in building more complex models and attempting to refine the estimates. You can only go so far with open-source material.

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Well, I brought that up because theoretically instead of having to do a wide scan to detect a fighter, with AESA you can focus the beam and sweep it rapidly so that you can improve the intensity of the beam without hurting coverage. Furthermore, most of these fighters don't operate in a vacuum. They'll have AWACs sweeping through different frequency bands trying to find the best points for a fighter to focus its radar beam in order to achieve effective BVR?

I understand that in the interest of parsimony these factors are dropped from your model, but I believe these are important factors to consider when we talk about how detectable two stealth fighters will be towards each other.

A New Photo Of The First Flight?

A.Man said:

A New Photo Of The First Flight?

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Come on man, you gotta know it's a PS.... and a crap one at that.

it is not P.S. the pic. is genuine

Bltizo said:

Come on man, you gotta know it's a PS.... and a crap one at that.

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Funny how I posted a warning about Zhonghua and Tiexue on the WS-10 thread just a few minutes ago.